Low Radio Frequency Loss, Static Dissipative Adhesives

ABSTRACT

An adhesive including an amorphous carbon dispersed in a cyanate ester resin, wherein the adhesive has static dissipative properties and low radio frequency loss properties.

FIELD

The present patent application relates to synthetic resins and, moreparticularly, to low radio frequency loss, static dissipative adhesives.

BACKGROUND

Due to random electro-static discharges, all metal parts on a spacecraftare typically grounded to bleed off static charges, thereby avoidingcatastrophic damage to the surrounding electronic hardware. For example,a space radar antenna is typically comprised of a plurality of radiofrequency (“RF”) radiating elements, commonly referred to as patches,that are supported by lightweight foam tiles. These patches, which aretypically made from metal, must be grounded.

One known technique for grounding RF radiating patches to lightweightfoam tiles involves the use of conductive mechanical fasteners, such asmetal pins. Such conductive mechanical fasteners secure the patches tothe lightweight foam tiles while at the same time providing a conductivepath for dissipating static charges. However, conductive mechanicalfasteners add complexity to the design and are not practical for usewith non-conductive lightweight foam tiles.

An alternative technique for grounding RF radiating patches tolightweight foam tiles involves the use of conductive adhesives.Conductive adhesives are attractive because they simplify themanufacturing process due to their ease of use. Furthermore, likeconductive metal fasteners, they secure (i.e., bond) the patches to thelightweight foam tiles while at the same time providing a conductivepath for dissipating static charges. However, the large quantities(e.g., 5 to 10 percent by weight) of conductive fillers, such as carbonpowder, graphite, ceramic and metal, in conductive adhesives impart theadhesives with high RF loss properties and, therefore, cannot be used inapplications, such as space radar antenna, that require low RF loss.

Accordingly, there is a need for a static dissipative adhesive that haslow RF loss properties.

SUMMARY

In one aspect, the disclosed low loss static dissipative adhesive mayinclude an amorphous carbon dispersed in a cyanate ester resin.

In another aspect, the disclosed low loss static dissipative adhesivemay include a quantity of carbon black dispersed in a cyanate esterresin, the quantity being such that the carbon black comprises at leastabout 0.1 percent by weight of the adhesive and at most about 2 percentby weight of the adhesive.

Other aspects of the low loss static dissipative adhesives of thepresent disclosure will become apparent from the following description,the accompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical illustration of return loss versus frequency foran exemplary low loss static dissipative adhesive in accordance with thepresent disclosure; and

FIG. 2 is a graphical illustration of insertion loss versus frequencyfor the exemplary low loss static dissipative adhesive of FIG. 1.

DETAILED DESCRIPTION

It has now been discovered that adhesives prepared by dispersingrelatively small quantities of amorphous carbon in a cyanate ester resinexhibit desired electrical conductivity and low RF loss properties.

In one aspect, the amorphous carbon useful in the formulations of thepresent disclosure may be any amorphous carbon having a relatively lowaverage particle size and relatively high surface area. Those skilled inthe art will appreciate that typical low average particle sizes foramorphous carbon may range from several microns to several nanometers.For example, the amorphous carbon in the formulations of the presentdisclosure may have an average particle size of about 1000 nanometers orless, or 500 nanometers or less, or 250 nanometers or less, or 100nanometers or less, or 50 nanometers or less, or 30 nanometers or less.

In one particular aspect, the amorphous carbon useful in theformulations of the present disclosure may be carbon black. An exampleof a useful and commercially available carbon black is PRINTEX® XE2,which is available from Evonik Industries (formerly Degussa) of Essen,Germany. PRINTEX® XE2 carbon black has an average particle size of about30 nanometers and a surface area of about 910 square meters per gram.Other useful commercially available carbon black pigments will bereadily apparent to those skilled in the art.

At this point, those skilled in the art will appreciate that the averageparticle size of commercially available amorphous carbons may be furtherreduced by mechanical processing. For example, PRINTEX® XE2 carbon blackmay be milled with ceramic media to achieve an average particle size ofless than 30 nanometers.

The cyanate ester resins useful in the formulations of the presentdisclosure may be any thermosetting resin wherein the hydrogen atom of ahydroxyl group (—OH) is substituted with a cyanide group (—CN) to form acyanate ester group (—OCN). For example, the cyanate ester resins may bebisphenol-A-based or novolac-based cyanate ester resins. An example of auseful and commercially available cyanate ester resin is EX-1515 resin,which is available from TenCate Advanced Composites USA, Inc. of MorganHill, Calif.

The disclosed low loss static dissipative adhesives may be prepared bydispersing the amorphous carbon in the cyanate ester resin. A solvent,such as methyl ethyl ketone or toluene, may be used to reduce theviscosity of the resin to assist in dispersing the amorphous carbontherein. Furthermore, solvent may be used to adjust the viscosity of theresulting adhesive depending on whether a liquid adhesive, a pasteadhesive or a film adhesive is desired.

The amount of amorphous carbon in the adhesive may depend on designconsiderations, including the required amount of electrical conductivityrequired and the amount of RF loss that may be tolerated. In one aspect,the disclosed adhesives may include at least about 0.1 percent by weightamorphous carbon and at most about 2 percent by weight amorphous carbon.In another aspect, the disclosed adhesives may include at least about0.1 percent by weight amorphous carbon and at most about 1.5 percent byweight amorphous carbon. In another aspect, the disclosed adhesives mayinclude at least about 0.1 percent by weight amorphous carbon and atmost about 1.0 percent by weight amorphous carbon. In another aspect,the disclosed adhesives may include at least about 0.1 percent by weightamorphous carbon and at most about 0.5 percent by weight amorphouscarbon.

At this point, those skilled in the art will appreciate that additionalcomponents may be included in the adhesive formulation, includingadditional pigments and resins, without departing from the scope of thepresent disclosure. Furthermore, a substrate material, such as fiber,may also be added to the adhesive formulation to form a compositematerial without departing from the scope of the present disclosure.

Specific examples of the disclosed low loss static dissipative adhesiveare set forth below.

EXAMPLES 1-8

Eight adhesives were prepared by dispersing various quantities ofPRINTEX® XE2 carbon black in EX-1515 cyanate ester resin. Solvent wasadded to the formulation to reduce the viscosity of the adhesives suchthat the adhesives may be sprayed with a pneumatic spray gun. Theresulting adhesives were then either sprayed or poured onto a D-shapedcomb pattern circuit having a 20 mil spacing and the electricalresistance was measured. The results are set forth in Table 1 below:

TABLE 1 Percent Electrical Carbon Application Resistance Example (byweight) Method (Ohms) 1 1.20 Sprayed <10⁵ 2 0.80 Sprayed <10⁵ 3 0.55Sprayed <10⁵ 4 0.50 Poured <10⁵ 5 0.40 Sprayed <10⁵ 6 0.40 Poured <10⁵ 70.28 Sprayed 2.2 × 10⁵ 8 0.13 Sprayed 6.0 × 10⁸

Thus, Examples 1-8 confirm that adhesives prepared with relatively lowquantities (e.g., less than 1 percent by weight) amorphous carbon incyanate ester resin provided electrical conductivity sufficient toeffectively dissipate static charges.

EXAMPLES 9-19

Eleven adhesives were prepared by dispersing various quantities ofPRINTEX® XE2 carbon black in EX-1515 cyanate ester resin. Solvent wasadded to the formulation to reduce the viscosity of the adhesives suchthat the adhesives may be sprayed with a pneumatic spray gun. Theresulting adhesives were then either sprayed or poured onto a trace totrace circuit having 15 millimeter spacing and the electrical resistancewas measured. The results are set forth in Table 2 below:

TABLE 2 Percent Electrical Carbon Application Resistance Example (byweight) Method (Ohms) 9 1.20 Sprayed 3.0 × 10⁵ 10 0.80 Sprayed 4.5 × 10⁵11 0.55 Sprayed 1.2 × 10⁶ 12 0.50 Poured 1.2 × 10⁶ 13 0.40 Sprayed 1.1 ×10⁷ 14 0.40 Poured 8.0 × 10⁶ 15 0.28 Sprayed 1.6 × 10⁷ 16 0.28 Sprayed1.4 × 10⁷ 17 0.28 Sprayed 1.2 × 10⁷ 18 0.28 Sprayed 1.3 × 10⁷ 19 0.13Sprayed  3.0 × 10¹⁰

Thus, Examples 9-19 also confirm that adhesives prepared with relativelylow quantities (e.g., less than 1 percent by weight) amorphous carbon incyanate ester resin provided electrical conductivity sufficient toeffectively dissipate static charges.

EXAMPLE 20

Using an appropriate mixer, 1 gram of PRINTEX® XE2 carbon black wasdispersed in 249 grams of EX-1515 cyanate ester resin to yield anadhesive having 0.40 percent by weight carbon black. The viscosity ofthe adhesive was adjusted and the resulting formulation was sprayed ontoa DI-STRATE™ 95225 syntactic foam tile having a thickness of 0.1375inches. DI-STRATE™ 95225 syntactic foam tiles are commercially availablefrom Aptek Laboratories, Inc. of Valencia, Calif.

The adhesive coated tile had an average dielectric constant of about 1.4and an average loss tangent of about 0.005 measured at 2 gigahertz whentested at both a vertical and horizontal polarization. The return lossversus frequency of the tile is shown in FIG. 1 and the insertion lossversus frequency of the tile is shown in FIG. 2.

EXAMPLE 21

Using an appropriate mixer, 1 gram of PRINTEX® XE2 carbon black wasdispersed in 356 grams of EX-1515 cyanate ester resin to yield anadhesive having 0.28 percent by weight carbon black. The viscosity ofthe adhesive was adjusted and the resulting formulation was sprayed ontoa DI-STRATE™ 95225 syntactic foam tile having a thickness of 0.1339inches.

At a vertical polarization, the adhesive coated tile had an averagedielectric constant of about 1.4 and an average loss tangent of about0.004 measured at 2 gigahertz. At a horizontal polarization, theadhesive coated tile had an average dielectric constant of about 1.4 andan average loss tangent of about 0.005 measured at 2 gigahertz.

EXAMPLE 22

The adhesive of Example 21 was sprayed onto a DI-STRATE™ 95225 syntacticfoam tile having a thickness of 0.1367 inches.

At both a vertical and horizontal polarization, the adhesive coated tilehad an average dielectric constant of about 1.4 and an average losstangent of about 0.005 measured at 2 gigahertz.

Accordingly, adhesives prepared by dispersing relatively smallquantities of amorphous carbon in a cyanate ester resin pursuant to thepresent disclosure exhibit desired electrical conductivity and low RFloss properties.

Although various aspects of the disclosed low loss static dissipativeadhesives have been shown and described, modifications may occur tothose skilled in the art upon reading the specification. The presentapplication includes such modifications and is limited only by the scopeof the claims.

1. An adhesive comprising an amorphous carbon dispersed in a cyanateester resin.
 2. The adhesive of claim 1 wherein said amorphous carbon iscarbon black.
 3. The adhesive of claim 1 wherein said amorphous carbonhas an average particle size of at most about 25 microns.
 4. Theadhesive of claim 1 wherein said amorphous carbon has an averageparticle size of at most about 1 micron.
 5. The adhesive of claim 1wherein said cyanate ester resin is a bisphenol-A-based cyanate esterresin.
 6. The adhesive of claim 1 further comprising a solvent.
 7. Theadhesive of claim 1 formulated as a liquid, paste or film.
 8. Theadhesive of claim 1 wherein said amorphous carbon comprises at mostabout 2 percent by weight of said adhesive.
 9. The adhesive of claim 1wherein said amorphous carbon comprises at most about 1.5 percent byweight of said adhesive.
 10. The adhesive of claim 1 wherein saidamorphous carbon comprises at most about 1 percent by weight of saidadhesive.
 11. The adhesive of claim 1 wherein said amorphous carboncomprises at most about 0.5 percent by weight of said adhesive.
 12. Theadhesive of claim 1 wherein said amorphous carbon comprises at leastabout 0.1 percent by weight of said adhesive.
 13. The adhesive of claim1 wherein said amorphous carbon is present in a quantity sufficient toimpart said adhesive with an electrical resistance of at most about 10⁹Ohms.
 14. The adhesive of claim 1 wherein said amorphous carbon ispresent in a quantity sufficient to impart said adhesive with adielectric constant of about 1.3 to about 1.7 and a loss tangent of atmost about 0.006, wherein said dielectric constant and said loss tangentare measured at about 2 gigahertz.
 15. The adhesive of claim 1 whereinsaid amorphous carbon is present in a quantity sufficient to impart saidadhesive with an electrical resistance of at most about 10⁷ Ohms, adielectric constant of about 1.3 to about 1.7 and a loss tangent of atmost about 0.006, wherein said dielectric constant and said loss tangentare measured at about 2 gigahertz.
 16. An adhesive comprising a quantityof carbon black dispersed in a cyanate ester resin, said quantity beingsuch that said carbon black comprises at least about 0.1 percent byweight of said adhesive and at most about 1 percent by weight of saidadhesive.
 17. The adhesive of claim 16 formulated as a liquid, paste orfilm.
 18. The adhesive of claim 16 wherein said carbon is present in aquantity sufficient to impart said adhesive with an electricalresistance of at most about 10⁹ Ohms.
 19. The adhesive of claim 16wherein said carbon black is present in a quantity sufficient to impartsaid adhesive with a dielectric constant of about 1.3 to about 1.7 and aloss tangent of at most about 0.006, wherein said dielectric constantand said loss tangent are measured at about 2 gigahertz.
 20. Theadhesive of claim 16 wherein said amorphous carbon is present in aquantity sufficient to impart said adhesive with an electricalresistance of at most about 10⁹ Ohms, a dielectric constant of about 1.3to about 1.7 and a loss tangent of at most about 0.006, wherein saiddielectric constant and said loss tangent are measured at about 2gigahertz.